Optical and electrical properties of porous silicon layer formed on the textured surface by electrochemical etching

Porous silicon(PS) layers were formed on textured crystalline silicon by electrochemical etching in HF-based electrolyte.Optical and electrical properties of the TMAH textured surfaces with PS formation are studied. Moreover,the influences of the initial structures and the anodizing time on the optical and electrical properties of the surfaces after PS formation are investigated.The results show that the TMAH textured surfaces with PS formation present a dramatic decrease in reflectance.The longer the anodizing time is,the lower the reflectance.Moreover,an initial surface with bigger pyramids achieved lower reflectance in a short wavelength range.A minimum reflectance of 3.86%at 460 nm is achieved for a short anodizing time of 2 min.Furthermore,the reflectance spectrum of the sample,which was etched in 3 vol.%TMAH for 25 min and then anodized for 20 min,is extremely flat and lies between 3.67%and 6.15%in the wavelength range from 400 to 1040 nm.In addition,for a short anodizing time,a slight increase in the effective carrier lifetime is observed.Our results indicate that PS layers formed on a TMAH textured surface for a short anodization treatment can be used as both broadband antireflection coatings and passivation layers for the application in solar cells.     

Nano‐cones on micro‐pyramids: modulated surface textures for maximal spectral response and high‐efficiency solar cells

The front‐side reflection represents a significant optical loss in solar cells. One way to minimize this optical loss is to nano‐texture the front surface. Although nano‐textured surfaces have shown a broad‐band anti‐reflective effect, their light scattering and surface passivation properties are found to be generally worse than those of standard micro‐textured surfaces. To overcome these setbacks in crystalline silicon solar cells, advanced texturing and passivation approaches are here presented. In the first approach, we propose a modulated surface texture by superimposing nano‐cones on micro‐pyramidal surface texture. This advanced texture applied at the front side of crystalline silicon wafers completely suppresses the reflection in a broad wavelength range from 300 nm up to 1000 nm and efficiently scatters light up to 1200 nm. In the second approach, we show a method to minimize recombination at nano‐textured surfaces by using defect‐removal etching followed by dry thermal oxidation. These two approaches are applied here in an interdigitated back‐contacted crystalline silicon solar cell and result in decoupling of the interplay between the mechanisms behind short‐circuit current density and open‐circuit voltage. The device exhibits a conversion efficiency equal to 19.8%, record external quantum efficiency (78%) at short wavelengths (300 nm), and electrical performance equal to the performance of the reference interdigitated back‐contacted device based on front‐side micro‐pyramids. Copyright © 2015 John Wiley & Sons, Ltd.     

多孔硅PL谱的影响因素分析

通过阳极氧化电化学方法制备了多孔硅,并在室温下对不同条件下制得的多孔硅光致发光谱(PL谱)进行系统的分析.结果表明,随着阳极电流密度、阳极化溶液浓度和时间的增大,多孔硅的PL谱峰将发生"蓝移",并且PL峰强也显著增加,但过大的电流密度、阳极化溶液浓度和时间将导致PL峰强下降.另外,还发现PL谱存在多峰结构,而多孔硅在空气中放置时间的延长将引起其PL的短波峰"蓝移"和强度下降,但对长波峰只引起强度减弱,并不影响其峰位.PL谱的多峰结构可以认为是由于样品中同时存在"树枝"状和"海绵"状两种微观结构所产生的,在这个假设下,用多孔硅氧化后发光中心从硅表面移到二氧化硅层及量子限制模型能够解释上述现象.     

超声波辅助湿法腐蚀制备多晶Si低反射表面

为降低光在多晶Si表面的反射,一种以湿法腐蚀为基础,添加超声波震荡的新方法,首次被用来腐蚀多晶Si太阳能电池片.利用扫描电子显微镜观察发现,此法所制备的多晶Si片表面形成了较多窄而深的沟壑状结构.其反射谱测试结果表明,此方法所获得的凹凸表面的减反射效果良好,加权反射率达到3.5%;同时减反射效果对入射光波长选择性不明显.最后,在实验的基础上对腐蚀机理进行了深入的探讨,认为超声波的空泡特性有利于腐蚀的纵向深入,抑制了腐蚀的横向发展,是形成此沟壑状形貌的主要原因.     

Texturization of mono-crystalline silicon solar cells in TMAH without the addition of surfactant

Etching was performed on(100) silicon wafers using silicon-dissolved tetramethylammonium hydroxide (TMAH) solutions without the addition of surfactant.Experiments were carried out in different TMAH concentrations at different temperatures for different etching times.The surface phenomena,etching rates,surface morphology and surface reflectance were analyzed.Experimental results show that the resulting surface covered with uniform pyramids can be realized with a small change in etching rates during the et...     

Reorganized Porous Silicon Bragg Reflectors for Thin-Film Silicon Solar Cells

Stacks of porous silicon layers have been successfully applied to maximize internal reflection at the interface between a silicon substrate and an epitaxially grown layer. The stack is consist of alternating porous layers of high and low porosity, defined by the quarter-wavelength rule. During the hydrogen bake prior to epitaxial growth of the epitaxial layer, the porous silicon stack crystallizes in the form of thin quasi-monocrystalline silicon layers incorporating large voids. Experimental data of the measured external reflectance have been linked to the internal reflectance. An optical-path-length enhancement factor of seven was calculated in the wavelength range of 900–1200 nm. Application on thin-film epitaxial solar cells showed a 12% increase in short-circuit current and efficiency.     

Texturization of mono-crystalline silicon solar cells in TMAH without the addition of surfactant

Texturization of mono-crystalline silicon solar cell by chemical anisotropic etching is still a key issue due to metal ions contamination and consumption of large amount of isopropyl alcohol (IPA) in a conventional mixture of potassium hydroxide (KOH) or sodium hydroxide (NaOH) and IPA. In this study, etching was performed on (100) silicon wafers using silicon-dissolved tetramethylammonium hydroxide (TMAH) solutions without addition of surfactant. Experiments were carried out in different TMAH concentration solutions at different temperatures for different etching time. The surface phenomena, etching rates, surface morphology and surface reflectance have been analyzed. Experimental results show that the resulted surface covered with uniform pyramids can be realized due to small changes of etching rates during the etching process. The etching mechanism has been explained basing on the experimental results and the theoretical considerations. It was suggested that all the components in the TMAH solutions play important roles in the etching process. Moreover, TMA+ ions may increase the wettability of the textured surface. A good textured surface can be obtained on conditions that the absorption of OH- /H2O is equilibrium with that of TMA+/SiO2(OH)22-.     

In situ nanoscale refinement by highly controllable etching of the(111) silicon crystal plane and its influence on the enhanced electrical property of a silicon nanowire

Nanoscale refinement on a(100) oriented silicon-on-insulator(SOI) wafer was introduced by using tetra-methyl-ammonium hydroxide(TMAH,25 wt%) anisotropic silicon etchant,with temperature kept at 50℃to achieve precise etching of the(111) crystal plane.Specifically for a silicon nanowire(SiNW) with oxide sidewall protection,the in situ TMAH process enabled effective size reduction in both lateral(2.3 nm/min) and vertical (1.7 nm/min) dimensions.A sub-50 nm SiNW with a length of microns with uniform triangular cross-section was achieved accordingly,yielding enhanced field effect transistor(FET) characteristics in comparison with its 100 nm-wide pre-refining counterpart,which demonstrated the feasibility of this highly controllable refinement process. Detailed examination revealed that the high surface quality ofthe(111) plane,as well as the bulk depletion property should be the causes of this electrical enhancement,which implies the great potential of the as-made cost-effective SiNW FET device in many fields.     

亚硝酸钠刻蚀液对多晶硅表面陷阱坑形貌的影响

酸刻蚀多晶硅表面技术是当前太阳能研究的热点之一。利用亚硝酸钠比硝酸钠氧化能力弱的特点,在普通酸刻蚀液中用亚硝酸钠取代硝酸配制多晶硅表面刻蚀液,然后在相同的工艺条件下刻蚀多晶硅表面。实验样品的SEM显示:含有NaNO2酸刻蚀液使多晶硅表面能布满蚯蚓状的腐蚀坑,腐蚀坑的深度比传统的酸刻蚀的陷阱坑深,而且密度分布比较均匀,样品平均反射率下降到23.5%,与传统配方酸刻蚀液刻蚀的多晶硅表面相比,平均反射率下降了8%左右。     

Sputter-Deposited AlTiO Thin Films for Semi-Transparent Silicon Thin Film Solar Cells

This paper reports on sputter-deposited AlTiO (ATO) thin films and their effects on the performance of semi-transparent silicon thin film solar cells. The electrical resistivity and the transparency of the ATO films depend significantly on the flow ratio of oxygen to argon during the reactive sputtering process. With highly transparent ATO films, transmittances of over 80% were obtained by increasing this flow ratio. When the ATO films were used on silicon substrates, they exhibited an anti-reflection property, where the minimum reflectance at visible light wavelength was decreased to 1.2%. The introduction of ATO thin film layers into solar cells resulted in a 24% increase in transmittance at wavelengths of around 700 nm, due to the film’s anti-reflection characteristic. In addition, the color of the cells changed from green to bright red as the ATO layers were adopted. These beneficial effects of the sputter-deposited ATO films suggest an effective pathway towards the semi-transparent silicon thin film solar cells for building-integrated photovoltaic system applications.     

Selective laser ablation of SiNx layers on textured surfaces for low temperature front side metallizations

For an alternative front side metallization process without screen printing of metal paste the selective opening of the front surface anti‐reflection coating could be realized by laser ablation. A successful implementation of this scheme requires direct absorption of the laser light within the anti‐reflection coating, since the emitter underneath must not be damaged severely. Additionally, the ablation must be feasible on textured surfaces. In this paper, we show that laser light with a wavelength of 355 nm and a pulse length of approximately 30 ns is absorbed directly by a typical silicon nitride anti‐reflection coating. Based on lifetime measurements on ablated samples it is shown that a damage free laser ablation of SiN_x layers on planar surfaces is possible. The characteristic ablation structure on textured surfaces is explained and quantified by rigorous coupled wave analysis (RCWA) simulations. Finally, high efficiency solar cells with a standard emitter (R_sh approx. 50 Ω/sq) have been processed using laser ablation of the silicon nitride anti‐reflection coating. These cells show efficiencies of up to 19·1%, comparable to the reference solar cells using photolithographically opened contact areas. Copyright © 2008 John Wiley & Sons, Ltd.     

硅基纳米结构的减反特性及其在太阳能电池中的应用

本文利用湿法化学腐蚀方法在硅基抛光衬底以及金字塔制绒的衬底上成功制备了纳米线阵列结构。在300~1000纳米波段,硅纳米线结构以及纳米线-金字塔混合结构都表现出了很好的减反特性,其平均反射率分别为2.53%、8%。利用传统工艺,我们在125mm125mm²的硅衬底上成功制备了短路电流密度为34.82mA/cm,开路电压为 594mv,效率为12.45%的纳米线太阳能电池。我们发现钝化对纳米结构的太阳能电池很重要,沉积钝化层之后可以将开路电压由420mv提高到560mv。我们通过分析所制备的太阳能电池的基本参数以及外量子效率,系统研究了硅基纳米结构太阳能电池的效率损失机制。实验证实钝化层以及电极的接触特性对提高纳米线太阳能电池的效率具有重要作用,并发现在已含PN结的硅衬底上制备纳米结构有助于提高太阳能电池的性能。     

Downward uniformity and optical properties of porous silicon layers

Porous silicon (PS) samples were fabricated by pulse current etching using different times. The downward uniformity and optical properties of the PS layers have been investigated using reflectance spectroscopy, photoluminescence spectroscopy, and scanning electron microscopy (SEM). The relationship between the refractive index and the optical thickness of PS samples and the etching depth has been analyzed in detail. As the etching depth increases, the average refractive index decreases, indicating that the porosity becomes higher, and the formation rate of the optical thickness decreases. Meanwhile, the reflectance spectra exhibit less intense interference oscillations,which mean the uniformity and interface smoothness of the PS layers become worse. In addition, the intensity of PL emission spectra is slightly increased.     

Optoelectronic study in porous silicon thin films

A photoconductivity (PC) study in as deposited porous silicon (PS) thin films is presented in this work. PS thin films were produced by the electrochemical anodizing method at different anodizing times. The films surfaces were characterized by SEM and porosity was determined by gravimetric methods. Photoluminescence and PC measurements were taken at room temperature. The maximum of the photoluminescence spectra are located around 650 nm, whereas those of PC are placed around 400 nm. The maximum of the photoluminescence signal shifts toward short wavelengths as the quantum dimension of the material skeleton diminishes, while any spectral displacement of the photocurrent signal as the porosity of the material increases is not observed. The spectral position of the PC signal does not change because it is strongly affected by the large quantity of defects present in the sample surface which diminishes the mean free path of the carriers to reach the electrodes. In all the samples photocurrent is small around 10^?1 μA and the intensity of the signal goes down as the porosity increases. Two mechanisms exist that compete with one another, the carrier generation and recombination through light emission centers which diminish the photocurrent.     

多孔硅薄膜的纵向均匀性及其光学特性研究

多孔硅样品使用脉冲电化学腐蚀法经过不同的腐蚀时间制备完成,使用反射光谱、光致发光光谱和SEM对多孔硅薄膜的纵向均匀性以及其光学特性进行了研究,还详细研究了随腐蚀深度变化的折射率和光学厚度(n*d)等光学参数。实验表明：随着腐蚀深度的增加,多孔硅薄膜的平均折射率n降低,即多孔度变大;多孔硅薄膜的光学厚度的形成速度减小;同时,反射光谱表现更弱的干涉性,表明薄膜的均匀性和界面的平整性变差;另外,光致发光谱的强度微弱变强。     

Effects of post-annealing treatment on the structure and photoluminescence properties of CdS/PS nanocomposites prepared by sol-gel method

CdS nanocrystals have been successfully grown on porous silicon (PS) by sol-gel method. The plan-view field emission scanning electron microscopy (FESEM) shows that the pore size of PS is smaller than 5 μm in diameter and the agglomerates of CdS are broadly distributed on the surface of PS substrate. With the increase of annealing time, the CdS nanoparticles grow in both length and diameter along the preferred orientation. The cross-sectional FESEM images of ZnO/PS show that CdS nanocrystals are uniformly penetrated into all PS layers and adhere to them very well. photoluminescence (PL) spectra demonstrate that the intensity of PL peak located at about 425 nm has almost no change after the annealing time increases. The range of emission wavelength of CdS/PS is from 425 nm to 455 nm and the PL intensity is decreasing with the annealing temperature increasing from 100 °C to 200 °C.     

Bulk micromachining of silicon in TMAH-based etchants for aluminum passivation and smooth surface

The fabrication of silicon based micromechanical sensors often requires bulk silicon etching after aluminum metallization. All wet silicon etchants including ordinary undoped tetramethyl ammonium hydroxide (TMAH)-water solution attack the overlaying aluminum metal interconnect during the anisotropic etching of (100) silicon. This paper presents a TMAH-water based etching recipe to achieve high silicon etch rate, a smooth etched surface and almost total protection of the exposed aluminum metallization. The etch rate measurements of (100) silicon, silicon dioxide and aluminum along with the morphology studies of etched surfaces are performed on both n-type and p-type silicon wafers at different concentrations (2, 5, 10 and 15%) for undoped TMAH treated at various temperatures as well as for TMAH solution doped separately and simultaneously with silicic acid and ammonium peroxodisulphate (AP). It is established through a detailed study that 5% TMAH-water solution dual doped with 38 gm/l silicic acid and 7 gm/l AP yields a reasonably high (100) silicon etch rate of 70 μm/h at 80 °C, very small etch rates of SiO₂ and pure aluminum (around 80 Å/h and 50 Å/h, respectively), and a smooth surface (±7 nm) at a bath temperature of 80 °C. The etchant has been successfully used for fabricating several MEMS structures like piezoresistive accelerometer, vaporizing liquid micro-thruster and flow sensor. In all cases, the bulk micromachining is carried out after the formation of aluminum interconnects which is found to remain unaffected during the prolonged etching process at 80 °C. The TMAH based etchant may be attractive in industry due to its compatibility with standard CMOS process.     

Texturization of silicon wafers with Na2CO3 and Na2CO3/NaHCO3 solutions for heterojunction solar-cell applications

The formation of pyramidal structures by anisotropic etching of 〈1 0 0〉-oriented monocrystalline silicon wafer surfaces is an effective method to reduce reflection losses originating on the front side of conventional silicon solar cells and silicon-heterojunction (SHJ) solar cells. One of the most common methods of texturization used in the solar-cell industry is based on aqueous solutions of NaOH or KOH and isopropyl alcohol (IPA). However, IPA is toxic and relatively expensive, so efforts are being made to replace it. Among the potential alternatives, solutions based on Na₂CO₃ and Na₂CO₃/NaHCO₃ mixtures have been proposed. In the present study, solutions of Na₂CO₃ and Na₂CO₃/NaHCO₃ mixtures were prepared in order to form pyramidal structures on silicon wafer surfaces. It was not possible to obtain uniform and completely textured surfaces by using aqueous solutions consisting only of Na₂CO₃. NaHCO₃ must be added in order to achieve uniform textured surfaces with low hemispherical reflectance suitable for SHJ solar-cell applications. Textured surfaces with good uniformity and low average hemispherical reflectance (15.4%) were prepared from 〈1 0 0〉 silicon substrates with relatively low etching times (25 min). Good surface passivation (lifetime >600 μs and implicit open-circuit voltage of 690±10 mV) on these p-type textured wafers were achieved.     

Experimental testing of a random medium optical model of porous silicon for photovoltaic applications

We have developed a model for light propagation in porous silicon (PS) based on the theory of wave propagation in random media. The low porosity case is considered, with silicon being the host material assuming randomly distributed spherical voids as scattering particles. The specular and the diffuse part of the light could be determined and treated separately. The model is applied to the case in which porous silicon would be used as a diffuse back reflector in a thin‐film crystalline silicon solar cell realized in an ultrathin (1–3 μm) epitaxially grown Si layer on PS. Three layer structures (epi/PS/Si) have been fabricated by atmospheric pressure chemical vapor deposition (APCVD) of 150–1000 nm epitaxial silicon layers on silicon wafers of which 150–450 nm of the surface has been electrochemically etched. An excellent agreement is found between the experimentally measured reflection data in the 400–1000 nm wavelength range and those calculated using the proposed model. The values of the layer thickness agree, within a reasonable experimental error, with those obtained independently by cross sectional transmission electron microscopy (XTEM) analysis. This provides an experimental verification of the random medium approach to porous silicon in the low porosity case. The analysis shows that the epitaxial growth process has led to appreciable porosity decrease of an initially high porosity layer from about 60% to 20–30%. Copyright © 2001 John Wiley & Sons, Ltd.     

Highly transparent modulated surface textured front electrodes for high‐efficiency multijunction thin‐film silicon solar cells

To further increase the efficiency of multijunction thin‐film silicon (TF‐Si) solar cells, it is crucial for the front electrode to have a good transparency and conduction, to provide efficient light trapping for each subcell, and to ensure a suitable morphology for the growth of high‐quality silicon layers. Here, we present the implementation of highly transparent modulated surface textured (MST) front electrodes as light‐trapping structures in multijunction TF‐Si solar cells. The MST substrates comprise a micro‐textured glass, a thin layer of hydrogenated indium oxide (IOH), and a sub‐micron nano‐textured ZnO layer grown by low‐pressure chemical vapor deposition (LPCVD ZnO). The bilayer IOH/LPCVD ZnO stack guarantees efficient light in‐coupling and light trapping for the top amorphous silicon (a‐Si:H) solar cell while minimizing the parasitic absorption losses. The crater‐shaped micro‐textured glass provides both efficient light trapping in the red and infrared wavelength range and a suitable morphology for the growth of high‐quality nanocrystalline silicon (nc‐Si:H) layers. Thanks to the efficient light trapping for the individual subcells and suitable morphology for the growth of high‐quality silicon layers, multijunction solar cells deposited on MST substrates have a higher efficiency than those on single‐textured state‐of‐the‐art LPCVD ZnO substrates. Efficiencies of 14.8% (initial) and 12.5% (stable) have been achieved for a‐Si:H/nc‐Si:H tandem solar cells with the MST front electrode, surpassing efficiencies obtained on state‐of‐the‐art LPCVD ZnO, thereby highlighting the high potential of MST front electrodes for high‐efficiency multijunction solar cells. Copyright © 2015 John Wiley & Sons, Ltd.